1. Field of the Invention
The present invention relates to a hydrostatic continuously variable transmission including a hydraulic pump and a hydraulic motor connected to each other through a hydraulic closed circuit, and configured so that the capacity of the hydraulic pump or the hydraulic motor is variably controlled, to vary the speed of input rotation of the hydraulic pump and to take out the speed-varied rotation as output rotation of the hydraulic motor. More particularly, the present invention relates to a clutch device for making and interrupting the communication between a high pressure side oil passage and a low pressure side oil passage, constituting the hydraulic closed circuit in the hydrostatic continuously variable transmission.
2. Description of the Background Art
Various types of hydrostatic continuously variable transmissions have been known and put to practical use. One example of a commercial hydraulic continuously variable transmission was sold by Honda and marketed under the trademark “Hondamatic” on the 2001 Honda Fourtrax Foreman Rubicon™ TRX500 ATV.
Another example of a hydrostatic continuously variable transmission is disclosed in Japanese Laid-open Patent No. Hei 6-42446, proposed by the present applicant.
Other examples of known hydrostatic continuously variable transmissions can be found in Japanese Patent No. 2920772 and Japanese Laid-open Patent No. Hei 9-100909. The hydrostatic continuously variable transmission disclosed in these Patent Documents generally includes a swash plate plunger pump, a swash plate plunger motor, and a hydraulic closed circuit for connecting the discharge port and the suction port of the swash plate plunger pump to the suction port and the discharge port of the swash plate plunger motor, respectively. This transmission is configured so that a pump swash plate member is driven by the engine, wherein a pump cylinder and a motor cylinder are connected to each other and disposed on an output shaft, a motor swash plate member is restricted in rotation, and the motor swash plate angle can be variably regulated.
It has been also known that, in the hydrostatic continuously variable transmission configured as above, a clutch valve is provided for making and interrupting fluid communication between a high pressure side oil passage and a low pressure side oil passage constituting the hydraulic closed circuit. The magnitude of transmission of a rotational drive force from the hydraulic pump to the hydraulic motor is controlled using the clutch valve, or a clutch control is conducted for interrupting the transmission of the rotation. For example, in Japanese Laid-open Patent No. Hei 9-100909, an automatic clutch device using such a clutch valve is disclosed.
Meanwhile, the clutch valve provided in the hydrostatic continuously variable transmission has been configured so that its operation is controlled by an actuator, operated by hydraulic pressure generated from a hydraulic governor operated in conjunction with the engine rotation. The known clutch valve has had the problems of a relatively large structure and high cost, due to the need for the hydraulic governor, the actuator and the like.
In view of this, the automatic clutch device described in Japanese Laid-open Patent No. Hei 9-100909 is configured so that the hydraulic pressure in the low pressure side oil passage urges the clutch valve in the opening direction, and the hydraulic governor for increasing a supplied hydraulic pressure supplied to the low pressure side oil passage according a rise in the input rotating speed of the hydraulic pump is connected thereto, to thereby eliminate the need for an exclusive-use actuator, and to provide a simplified structure. However, since the operation of the clutch valve is controlled by the hydraulic pressure using the hydraulic governor, this limits the manufacturer's ability to make the device smaller and compact and to limit the cost. A need still exists for a device which a further simplifies the structure of a clutch for this type of transmission.